The usual pressure gauges on the dashboards of automotive vehicles emit a visual and somtimes an audible signal when the oil pressure falls below a predetermined threshold. Since, however, this oil pressure tends to increase with engine speed and thus is relatively low under idling conditions, the threshold must be set at a value lower than the minimum idling pressure required in order to avoid the untimely emission of an alarm signal. The rationale for this arrangement is that a sufficient oil pressure at minimum engine speed will insure adequate lubrication also at higher speeds when that pressure is automatically intensified by the acceleration of the engine-driven oil pump.
With a faulty lubricating system, or in the presence of an insufficient amount of oil in the engine, it could still happen under load (e.g. when the vehicle passes through a curve) that the oil pressure drops to a value above the threshold but insufficient for satisfactory lubrication at higher speeds. Thus, the driver may be unaware of a situation which could be damaging to the engine; even if an alarm signal appears at that point, damage may result since the operator may be unable to decelerate the vehicle quickly enough.
To avoid these inconveniences, certain gauges have already been developed which are designed to indicate the amount of oil circulating in the lubricating system. These gauges, however, do not function very accurately when the vehicle is in motion since frequent changes in speed and direction maintain the oil in a state of turbulence.